A particular form of Viral Marketing.
You have a science fiction movie coming out. What can do you do to promote it that goes that extra mile?
You do In-Universe Marketing. You post "found" documents from within the universe. A "NBS Nightly News" broadcast from 1972 in your world? Great. Advertising a drink from your setting? Yup. Showing a half hour of dull security camera footage with maybe thirty seconds of profoundly disturbing stuff going on? Perfect. The diary of one of the characters, with pages torn out? Synergylicious!
Examples should go under Viral Marketing; this entry just defines the term.